Astronomers have observed Sagittarius A* – the supermassive black hole at the centre of our galaxy – closer than ever before, and Albert Einstein’s general theory of relativity still holds up
14 December 2021
We have seen the area close to our galaxy’s black hole more clearly than ever before. These observations have shown that, so far, Albert Einstein’s general theory of relativity – which dictates how gravity affects space-time – remains accurate.
One way to test general relativity is to look at the motion of stars around a supermassive black hole, where the effects of gravity are more extreme than anywhere else in the universe. Reinhard Genzel at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, and his colleagues used the Very Large Telescope Interferometer in Chile to expand on his Nobel-winning research on the supermassive black hole at the centre of the Milky Way, called Sagittarius A*.
Their observations, which combined data from four telescopes in a technique called interferometry, were about 20 times sharper than they could have been with a single telescope. “What this allows us to do is to look deeper and see fainter sources with this high resolution,” says team member Julia Stadler, also at the Max Planck Institute for Extraterrestrial Physics.
By measuring the speeds of the stars orbiting Sagittarius A*, the researchers calculated that the black hole has a mass about 4.30 million times that of the sun, the most precise measurement of its mass yet. Those stars all seem to be obeying the predictions of general relativity.
“We have not found any surprises in the sense of general relativity,” says Genzel. “But we are starting to really test out various proposals which are extensions of the theory.”
Genzel and his colleagues found a new star orbiting Sagittarius A* that could help us test these alternative proposals. Called S300, it is older and fainter than the other stars we have seen in the area, and its age could change its light in a way that would make it possible to measure its motion with extraordinarily high precision.
Its discovery also gives us hope that there might be faint stars even closer to the black hole than we thought, says Genzel. Those stars would provide even better testing grounds for general relativity, and the researchers are now working on figuring out how to find them.
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